This application aims to develop a novel strategy for preventing chronic myeloid leukemia (CML) by targeting leukemia stem cells (LSCs) that initiate CML. This idea is based on our recently published finding that the arachidonate 5-lipoxygenase (5-LO) gene (Alox5) is a key survival-regulatory gene in LSCs (Chen et al. Nature Genetics 41:783-792, 2009), suggesting that Alox5 serves as a potential target gene for preventing CML. A known function of 5-LO is to produce inflammatory leukotrienes, and enzymatic activity of 5-LO can be inhibited by an FDA-approved human anti-inflammatory drug Zileuton (Zyflo). Thus, Zileuton could be a promising chemopreventive agent for CML; the underlying mechanisms are totally unknown and could be related to the effects of Zileuton on the known metabolic function or novel functions of Alox5 or both. Our preliminary studies also show that Zileuton suppresses human CML stem cells. Because CML is derived from a stem cell harboring the BCR-ABL oncogene, our findings prompt us to hypothesize that Zileuton acts on an Alox5-related molecular network responsible for survival regulation of LSCs and is a potential chemopreventive agent for CML. Testing this hypothesis is of clinical significance and would benefit: 1) individuals who have detectable BCR-ABL transcripts but have not developed CML; 2) CML patients who are in molecular remission and hope or have to stop a BCR-ABL kinase inhibitor; and 3) CML patients who had bone marrow transplantation but may still have residual leukemia cells. Mechanistically, because the known Alox5 function is to produce leukotrienes, Zileuton may prevent CML by reducing their production, which needs to be tested. Our preliminary data also reveal novel functions of Alox5, suggesting that Zileuton may suppress Alox5 function through regulating pathways unrelated to production of leukotrienes. In this regard, our preliminary study in shows that loss of P-selectin causes upregulation of Hif1a which enhances Alox5 expression in LSCs and that Alox5 signals to beta-catenin. Inhibition of LSCs by Zileuton provides a novel preventive strategy for CML. In-depth study of Zileuton function helps to identify more Alox5-related target genes for preventing CML and provides a rationale for modifying existing drugs with activities on Alox5 and related genes. The specific aims are: 1) To determine whether leukotrienes promote the survival of LSCs, whether Zileuton suppresses LSCs through reducing leukotriene production and what the underlying mechanism is; 2) To determine the intracellular pathways affected by Zileuton in LSCs; and 3) To determine whether Zileuton similarly regulates the Alox5 pathway in human CML stem cells and reduces engraftment of human CML cells in immunocompromised mice. Although we focus on CML, Zileuton is also a potential chemopreventive agent for solid tumors as Alox5 is associated with carcinogenesis in the colon, lung, pancreas, liver, esophagus, skin.